This invention relates to measurement of the speed of a rotating shaft, and more particularly to a method and apparatus for developing an electrical signal whose frequency is indicative of the shaft speed for either direction of rotation and is not zero at zero shaft speed.
The output signals from conventional dc and ac tachometers approach zero level and zero frequency when the rotating shaft speed approaches zero. With the shaft turning very slowly, the output dc level of a dc tachometer is very low and noise and drift can produce significant changes in output signal that are not directly related to shaft speed. In the case of an ac tachometer, the output frequency and output voltage magnitude approach zero resulting in loss of signal and a long time constant associated with the low-frequency output. The output frequency is also the same for both directions of rotation. Of course, there are various other techniques for providing an electrical signal proportional to shaft speed. For applications such as the sensing of rotor speed in an adjustable speed ac or dc motor reversing drive system, an improved technique for accurate speed measurement especially at and near zero speed is needed.